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The existence of genotype by environment interaction (G*E) can cause a change in the ranking of bulls tested in one environment when their daughters are subsequently milked in another environment. If the degree of re-ranking is large, the genetic correlation between milk production in the two environments will be substantially less than 1.0, with the implication that proofs made in one environment may not be reliable predictors of proofs in the second environment. The aim of this study was to investigate the effect of increasing the difference between environments, defined on the basis of herd average milk yield, on the genetic correlation (rg) for milk, fat and protein yield in Holstein Friesian dairy cattle in Ireland.